3-D Printed Microfluidic Sensor in SIW Technology for Liquids' Characterization

This article presents the implementation of a novel microfluidic sensor based on a resonant cavity. The cavity is based on the substrate integrated waveguide (SIW) technology and is fabricated by using stereolithographic 3-D printing, which guarantees fast prototyping and allows arbitrary geometry. The proposed sensor consists of a square cavity and an inner meander pipe provided with two openings, where the liquid under test is injected and extracted, and is fed by a coaxial probe, which represents the microwave port of the circuit. S-parameter measurement allows retrieving the electromagnetic properties of the liquid injected in the pipe. In particular, the loss tangent of the liquid is extracted from the variation of the cavity quality factor compared to air-filled pipe, after removing the actual losses of the structure. The dielectric permittivity of the liquid is retrieved from the shift of the cavity resonance frequency relative to the case of air-filled pipe, without any hypothesis of small losses. The performance of the sensor is discussed through the electromagnetic characterization of several fluids.